Which of these numbers is composite? ${17,\ 37,\ 61,\ 81,\ 97}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 17, 37, 61, and 97 each have only two factors. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 81 is the composite number.